This invention relates to improvements in lighting, particularly to warning lights for external use on aircraft.
External aircraft lights, broadly speaking, fall into two main categories. The first is for the purpose of illuminating the space around the aircraft to improve visibility for the pilot, of which landing lights are an example. The second is to act as warning lights for the purpose of providing a visible warning of the aircraft's presence to observers both on the ground and in other aircraft.
Essentially, there are two types of external aircraft warning lights: anti-collision lights and navigation lights.
Anti-collision lights are intended to attract the attention of observers, especially in low light conditions and, accordingly, these lights are designed to emit very bright light all around the aircraft and are usually pulsed so that they flash at between about 40 to 100 times a minute. In addition to the necessity of emitting light all around the aircraft, regulations imposed by the relevant national governing aviation bodies—such as the Civil Aviation Authority (CAA) in the UK and the Federal Aviation Authority (FAA) in the USA—stipulate that there should be a low divergence in the vertical plane. Accordingly, anti-collision lights ideally emit light in a substantially horizontal disk pattern.
Once an observer is made aware of the presence of an aircraft by its anti-collision lights, the navigation lights provide an indication of the orientation of the aircraft. Typically, an aircraft carries at least three navigation lights: a green light on the starboard side, a red light on the port side and a white light on the rear of the aircraft. For example, these may be located respectively on the end of the wings and on the tail of an aeroplane. They are less bright than anti-collision lights and are generally illuminated continuously in use.
In contrast with anti-collision lights which are required to be visible around 360° in a horizontal plane, both the horizontal and vertical distribution of emitted light from a navigation light are important. This is because each navigation light is required to emit light in a horizontal plane around the aircraft and at a minimum intensity which varies according to angular direction. For example, the red and green lights are not only required to emit bright light directly forward from the aircraft, but are also required to emit light to the port side and the starboard side respectively, albeit of a lower minimum intensity than in the forward direction.
In fact, required angular distributions of the FAA of light for a starboard navigation light are shown in FIG. 1. FIG. 1a shows the required azimuthal distribution and FIG. 1b shows the required angular distribution, in the vertical plane. It will be appreciated that the corresponding azimuthal distribution for a port light is merely the mirror image of that shown for the starboard light. The minimum light intensity at various azimuthal angles is shown (0° corresponding to the direction in which the aircraft faces). The FAA requires a minimum light intensity of 40 candela between 0° and 10°, 30 candela between 10° and 20° and 5 candela between 20° and 110°.
A variety of light sources are used in external aircraft lights, traditionally filament lamps and xenon flash tubes have been employed, but increasingly arrays of light emitting diodes (LEDs) are being used due to their greater reliability, robustness, lifetime and their low cost.
It will be appreciated that all these light sources emit uncollimated light that diverges over wide angles. As discussed, this can create problems, particularly for warning lights, where there is a need to satisfy the light emission regulations imposed by the relevant governing aviation authorities. Accordingly, it has been common practice to provide reflectors, for example, in order to divert the emitted light in the desired manner. However, use of rear reflectors is not an option when the LEDs are surface mounted as is usually the case.
As mentioned, conventional LEDs generally emit light with a smooth angular variation over a wide angle, so that it would appear approximately circular if included in FIGS. 1a and 1b. In fact, LED lamps emit light to about 60° on either side azimuthally of the forward direction. Thus in order to cover the required azimuthal emission range over 0° to 110°, port and starboard navigation lights must be turned away from the forward direction of the aircraft. They are typically pointed away from the forward direction, for example at about 30–35°, and, consequently, the brightest light is not emitted in the forward direction of the aircraft as would be desired. Consequently, the light intensity is too weak to meet the FAA required intensity in the forward direction.
Accordingly, a replacement bulb for a typical navigation light that includes both visible LEDs and IR LEDs for covert operations, such as that shown in WO98/21917 A1 to LFD Limited, would produce a diverging illumination pattern.
Collimation of light emitted by LEDs in an external aircraft light is known from GB 2,307,977 A1 to Chapman and Bloxham. In the particular embodiment disclosed, the aircraft light is a landing light comprising a central halogen lamp for producing a visible beam of light surrounded by a ring of IR LEDs. The landing light can be switched between normal and covert modes. Some of the IR LEDs have discrete plano-convex lenses associated with them, these lenses being individually mounted on a plate for the purpose of collimating the light emitted from the diode underneath into a narrow beam, whilst others are allowed to emit IR light in the normal way through corresponding apertures in the plate to provide area illumination for pilots wearing night vision goggles (NVGs).
However, Chapman and Bloxham merely collimates the light as it propagates through the lenses, and offers no suggestion of or assistance with directing the emitted light into a desired angular distribution.